In general, an image sensor is a semiconductor device for converting optical images into electrical signals, and is mainly classified as a charge coupled device (CCD) or a Complementary Metal Oxide Semiconductor (CMOS) image sensor.
CCD image sensors utilize a complicated driving scheme, consume large amounts of power, and require a multi-step photo process. This leads to the disadvantage of a complicated manufacturing process. CMOS image sensors have been looked to recently for overcoming the disadvantages of a CCD.
A CMOS image sensor sequentially detects electrical signals of pixels in a switching scheme by forming photodiodes and MOS transistors in unit pixels.
A CMOS image sensor utilizes a simpler driving scheme than the CCD image sensor. It is also capable of implementing a variety of scanning schemes and integrates signal processing in a single chip so that miniaturizing a product is possible. A CMOS image sensor uses CMOS technology so that manufacturing cost and power consumption are low.
A CMOS image sensor according to the related art has a substrate divided into a photodiode region for receiving a light signal and converting the light signal into an electrical signal, and a transistor region for processing the electrical signal.
A typical CMOS image sensor has a structure such that the photodiode and the transistor are horizontally arranged on the semiconductor substrate.
While the horizontal-type CMOS image sensor improves upon some of the disadvantages of the CCD image sensor, problems still exist in the horizontal-type image sensor.
According to the horizontal-type image sensor, the photodiode and the transistor are formed to be horizontally adjacent to each other on a substrate. Thus, the photodiode cannot encompass the entire area of a pixel region of the image sensor, leading to a reduced fill factor or limited resolution.
In addition, with a horizontal-type image sensor of the related art, it is very difficult to achieve optimization for a process simultaneously manufacturing the photodiode and the transistor. In particular, a shallow junction is required for a low sheet resistance in a rapid transistor process, but the shallow junction may not be appropriate for the photodiode.
Furthermore, according to the horizontal-type CMOS image sensor of the related art, additional on-chip functions are added to the image sensor so that the sizes of unit pixels are increased or reduced in order to maintain the sensitivity of the image sensor. When the size of the unit pixel is increased, the resolution of the horizontal-type CMOS image sensor decreases. In addition, when the area of the photodiode is decreased, the sensitivity of the horizontal-type CMOS image sensor decreases. Thus, there exists a need in the art for a CMOS image sensor and fabricating method with an improved fill factor.